It's really nothing special, Bar references Foo, Foo references PostAddress, all via composite key class. Since the foreign keys are composite and part of the PK, I must nest the ID classes into the ID classes. I thought this is correct. However, I get the following stack traces with Hibernate 3.6 and EclipseLink 2.2.0

Hibernate:

org.hibernate.AssertionFailure: Unexpected nested component on the referenced entity when mapping a @MapsId: tld.transmuc.model.Foo
at org.hibernate.cfg.CopyIdentifierComponentSecondPass.doSecondPass(CopyIdentifierComponentSecondPass.java:101)
at org.hibernate.cfg.Configuration.processSecondPassesOfType(Configuration.java:1424)
at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1388)
at org.hibernate.cfg.Configuration.buildMappings(Configuration.java:1345)
at org.hibernate.ejb.Ejb3Configuration.buildMappings(Ejb3Configuration.java:1477)
at org.hibernate.ejb.EventListenerConfigurator.configure(EventListenerConfigurator.java:193)
at org.hibernate.ejb.Ejb3Configuration.configure(Ejb3Configuration.java:1096)
at org.hibernate.ejb.Ejb3Configuration.configure(Ejb3Configuration.java:278)
at org.hibernate.ejb.Ejb3Configuration.configure(Ejb3Configuration.java:362)
at org.hibernate.ejb.HibernatePersistence.createEntityManagerFactory(HibernatePersistence.java:56)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:48)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:32)
at tld.transmuc.Main.main(Main.java:27)
Exception in thread "main" javax.persistence.PersistenceException: [PersistenceUnit: transmuc] Unable to configure EntityManagerFactory
at org.hibernate.ejb.Ejb3Configuration.configure(Ejb3Configuration.java:374)
at org.hibernate.ejb.HibernatePersistence.createEntityManagerFactory(HibernatePersistence.java:56)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:48)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:32)
at tld.transmuc.Main.main(Main.java:27)
Caused by: org.hibernate.AssertionFailure: Unexpected nested component on the referenced entity when mapping a @MapsId: tld.transmuc.model.Foo
at org.hibernate.cfg.CopyIdentifierComponentSecondPass.doSecondPass(CopyIdentifierComponentSecondPass.java:101)
at org.hibernate.cfg.Configuration.processSecondPassesOfType(Configuration.java:1424)
at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1388)
at org.hibernate.cfg.Configuration.buildMappings(Configuration.java:1345)
at org.hibernate.ejb.Ejb3Configuration.buildMappings(Ejb3Configuration.java:1477)
at org.hibernate.ejb.EventListenerConfigurator.configure(EventListenerConfigurator.java:193)
at org.hibernate.ejb.Ejb3Configuration.configure(Ejb3Configuration.java:1096)
at org.hibernate.ejb.Ejb3Configuration.configure(Ejb3Configuration.java:278)
at org.hibernate.ejb.Ejb3Configuration.configure(Ejb3Configuration.java:362)
... 4 more

(EDIT): Another issue is the use of the FooId. This class should not be used. The ID class for Foo Enitity is the PostAddressId and the OneToOne "postAddress" should simply be marked @Id and not @MapsId. Likewise BarId should not have an Embedded defined but should directly reference the PostAddressId. The Bar Mapping that references PostAddress should use MapsId.

Please file a bug against EclipseLink for the terrible exception and if removing the @Embedded and the other updates in the edit does not resolve your issue please file an EclipseLink bug for that as well.